Explosive Find: Double Stars Likely to Merge With a Bang

Below:

Next story in Space

Astronomers have discovered a dozen previously unknown
double-star systems, each on a course to end in spectacular
explosions detonated by the crash between their two small, dense
stars.

All of the newfound
binary star systems consist of two white dwarf stars. It is
these stars that are on a collision course in each system.

A white dwarf is the hot, dead core left over when a sun-like
star puffs off its outer layers as it dies. They are incredibly
dense, packing as much as a sun's worth of material into a sphere
the size of Earth. A teaspoon of it would weigh more than a ton.
[ Illustration
of merging white dwarf stars ]

"These are weird systems objects the size of the Earth orbiting
each other at a distance less than the radius of the sun," said
astronomer Warren Brown of the Harvard-Smithsonian Center for
Astrophysics in Cambridge, Mass., lead author of a pair of papers
reporting the find.

The astronomers discovered 12 binary white dwarf systems about
half of which will likely see their stars merge into one, then
explode.

The tightest binary system, one in which its two stars circle
each other once every hour, will merge in about 100 million
years, the scientists estimated.

The
white dwarfs found in this survey are relatively lightweight,
holding only about 20 percent as much mass as the sun. They are
made almost entirely of helium, unlike normal white dwarfs made
of carbon and oxygen.

"These white dwarfs have gone through a dramatic weight-loss
program," said Carlos Allende Prieto, an astronomer at the
Instituto de Astrofisica de Canarias in Spain and a co-author of
the study. "These stars are in such close orbits that tidal
forces, like those swaying the oceans on Earth, led to huge mass
losses."

Because they whirl around so close to each other, the white
dwarfs' gravity can stir space-time around them, creating
expanding ripples known as
gravitational waves. Those waves, which scientists have yet
to directly detect, are thought to carry away orbital energy and
cause the stars to spiral closer together.

Collision-course stars

The six newfound star systems with merging white dwarfs boost the
number known of double-star setups in the act of merging into
one."We have tripled the number of known, merging white-dwarf
systems," said co-author Mukremin Kilic, also of the
Harvard-Smithsonian Center for Astrophysics. "Now we can begin to
understand how these systems form and what they may become in the
near future."

When two white dwarfs merge, their combined mass can exceed a
tipping point, causing them to detonate and explode as a Type
Ia supernova.

Brown and his colleagues suggest that the merging binaries they
have discovered might be one source of less flashy supernovas a
rare type of supernova explosion 100 times fainter than a normal
Type Ia supernova, which ejects only 20 percent as much matter.

"The rate at which our white dwarfs are merging is the same as
the rate of under-luminous supernovae about one every 2,000
years," Brown said. "While we cant know for sure whether our
merging white dwarfs will explode as under-luminous supernovae,
the fact that the rates are the same is highly suggestive."

The paper announcing their find will be published in an upcoming
issue of the Monthly Notices of the Royal Astronomical Society.